Assessing the redox state of Mariana forearc

Subduction zones ("arcs") are the geological setting where one tectonic plate ("slab") is dragged underneath another into the Earth's deep interior ("mantle"). They are sites of major chemical exchange between the Earth's surface and interior. Water and other volatiles (e.g. sulfur, chlorine, fluorine, carbon) are released from the subducting slab in the form of fluids and transported to the overlying plate ("fore-arc mantle"). These fluids are considered to play a critical role in the transport and concentration of economically-important elements like copper and gold in subduction zones and the chemistry of arc magmas and associated volcanic gases. Deciphering the chemistry of these fluids is also fundamental to understanding the Earth's carbon and sulfur cycles and the evolution of the Earth's atmosphere and oceans. These cycles rely on knowing both the efficiency of carbon and sulfur burial by subduction, and the loss of these elements from the subducting slab to the fore-arc mantle. However there are major gaps in our knowledge of slab fluid chemistry and especially in relation to the redox cycling of sulfur, iron and carbon. The goal of this project is to constrain the nature of slab fluids and quantify the budgets and oxidation states of sulfur, carbon and iron in these fluids using a combination of novel chemical fingerprinting tools that we have demonstrated to be highly effective for this purpose.
In this project we will take advantage of the IODP expedition 366 to visit and sample the Mariana arc. The Marianas are one of the best-studied arcs, and one of the few places where we can directly access and sample the fore-arc mantle. This expedition presents a unique opportunity to study the zone where fluids are released from the subducting slab and react with the mantle of the overlying plate. From working on samples collected during this expedition we will be able to directly study the reactions between the fluids released from the subducting slab and the overlying fore-arc mantle and we will be able to use our results to determine the compositions and redox budgets of these fluids and to address their role in global carbon and sulfur cycles.

Industry - We aim to develop sulfur and iron K-edge measurements in phyllosilicates at the synchrotron. The development of synchrotron technics is vital for both academia and industry since these technics are commonly used in both sectors, as evidenced by collaborations between scientific companies (e.g. CEA - French Nuclear Energy Division) and synchrotrons. Sulfur and sulphides have also been identified by both NERC ('Sustainable use of Natural Resources' theme) and the European Union ('EU-14 critical raw materials') as materials on which the European economy depends, but which might be at risk of supply disruption. A better understanding of S redox cycle during fluid/rock interactions will be a key science output of this proposal. Moreover, the focus of the project on stable isotope systems such as Fe and Zn will contribute to establishing these systems as tracers within the exploration industry.
Scientific and intergovernmental bodies - IODP research promote scientific collaboration between scientists among the world and with different expertise. The participation to such type of expedition constitutes a great opportunity to develop international collaboration. In addition, the proposed research concerns the cycling of C in forearc settings. Carbon sequestration research is motivated by the concern that increasing atmospheric carbon dioxide concentrations will drive long-term change to Earth's climate (Intergovernmental Panel on Climate Change: https://www.ipcc.ch/report/srccs/).
Education - The cycling of redox-sensitive elements such as carbon, iron and sulfur during plate tectonics is fundamental to the development of the Earth as a habitable planet. Our work addresses this process on the Earth at the largest scale and has major implications for many natural phenomena associated with subduction that are of interest to the general public e.g. volcanism and volcanic degassing, ore-deposit formation and occurrence, plate tectonics and mountain-building processes. This provides us with an exciting opportunity to clarify the processes that form the basis of these phenomena and to feed this information back to the general public.